Magnesium production from a molten oxide slag in the presnce of hydrogen

ABSTRACT

MAGNESIUM IS PRODUCED BY THE REDUCTION OF MAGNESIUM OXIDE IN A MOLTEN SLAG AT ELEVATED TEMPERATURE WITH A MOLTEN METAL REDUCING AGENT, IN THE PRESENCE OF HYDROGEN AT A PRESSURE OF AT LEAST ABOUT ONE ATOMPSHERE.

United States Patent 3,761,247 MAGNESIUM PRODUCTION FROM A MOLTEN OXIDE SLAG IN THE PRESENCE OF HYDROGEN Julian Miles Avery, 47 Old Orchard Road, Chestnut Hill, Mass. 02167 No Drawing. Continuation-impart of applications Ser. No. 648,856, June 26, 1967, now Patent No. 3,579,326, Ser. No. 796,214, Feb. 3, 1969, now Patent No. 3,658,- 509, and Ser. No. 26,118, Apr. 6, 1970. This application May 17, 1971, Ser. No. 144,321 Int. Cl. C22b 45/00 US. Cl. 75--67 9 Claims ABSTRACT OF THE DISCLOSURE Magnesium is produced by the reduction of magnesium oxide in a molten slag at elevated temperature with a molten metal reducing agent, in the presence of hydrogen at a pressure of at least about one atmosphere.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my previous applications, Ser. Nos. 648,856, now US. Pat. No. 3,579,326, 796,214, now US. Pat. No. 3,658,509, and 26,118, filed June 26, 1967, Feb. 3, 1969 and Apr. 6, 1970, and is related to my previous applications Ser. Nos. 26,117, now US. Pat. No. 3,681,053, and 26,116, now US. Pat. No. 3,698,888, both filed Apr. 6, 1970 as well as my application Ser. No. 143,866 filed concurrently herewith.

BACKGROUND OF THE INVENTION I have for some time maintained that the most advantageous method of producing magnesium lies in the chemical reduction of magnesium oxide with a reducing agent in the presence of a molten oxide slag, in an electric furnace. Moreover, I have disclosed that such a process can be carried out at about atmospheric pressure by the use of inert gas in the vapor space of the reactor. The inert gases I have recommended include not only the literally inert gases, but also hydrogen and, more recently, methane.

I have pointed out several of the advantages which make hydrogen a preferred choice as the inert gas, but only recently have I come to recognize fully the considerable advantages, not immediately apparent, which render the use of hydrogen, at a pressure of at least about one atmosphere, superior to other choices. Hydrogen, I have found, not only is advantageous because of its relatively low cost and ready availability, but also for its high difi'usivity, specific heat, stability and low density, which render it capable of achieving maximum advantages in the processes of my related inventions.

Hydrogen has been studied for use in magnesium reduction, principally in "Germany before and after the Second World War, see W. Moschel et al., Magnesium, Chemische Technologie (Carl Hanser Verlag, Munich, 1953), pp. 102 et seq., but was eventually discarded in favor of vacuum operation after extensive testing, because of the necessity of higher operating temperature and greater quantities of raw materials, a conclusion borne out as well in this country (id. at 145-46). In operation, under high vacuum, hydrogen was used apparently only to fill the oven between runs, to facilitate the remelting of the condensed magnesium product (id. at 147). In these processes the magnesium oxide was reduced in the solid state, rather than in a molten slag. In the only commercial molten-slag process, Magnetherm, which has-been in full-scale operation for seven years,

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no use of hydrogen has been made, although argon is used to flush the furnace between runs, apparently performing a function analogous to the hydrogen in the German process above mentioned; see Artru et al., US. Pat. No. 2,971,833; also F. Trocme, The Development of the Magnetherm Process, 'Light Metals 1971 (Edgeworth, ed., AIME mtg., 1971). Representatives of Magnetherm have stated to me that they can see no advantage, and considerable potential danger, in using hydrogen in the molten slag furnace, over either their present vacuum system or over a pressurized system employing argon or helium.

Notwithstanding these denials, I believe and have recently been confirmed by independent authority, that the use of hydrogen above a molten slag at a pressure of at least one atmosphere in the reactor-condenser system can, Without danger, achieve substantial, unexpected process advantages which represent a major breakthrough in the art. It is, therefore, the major obejctive of this invention to provide a process for the reduction of magnesium oxide to magnesium by a reducing agent in a molten oxide slag, and the evolution and condensation of magnesium vapor, in the presence of hydrogen and at a pressure of at least about one atmosphere.

BRIEF SUMMARY OF THE INVENTION The present invention relates to a process for the production of magnesium by reduction of magnesium oxide in a molten slag and the evolution and condensation of magnesium vapor. In particular, the process is carried out in the presence of hydrogen and at a pressure of at least about one atmosphere.

DETAILED DESCRIPTION OF THE INVENTION In the present invention the reduction of magnesium oxide to magnesium and the evolution and condensation of magnesium vapor are conducted in the presence of hydrogen in an electric furnace-condenser system at a pressure of at least about one atmosphere. The maximum pressure is about five atmospheres, and the preferred range of pressure is about 1 to 2 atmospheres, although at intermittent periods, the pressure may be increased for special purposes, e.g. to assist exhausting molten magnesium from the condenser or spent slag from the reactor.

The source of magnesium oxide may be calcined dolomite, preferably including magnesia or calcined magnesite, as discussed in my related applications. I have also described in my related applications suitable and preferred reducing agents, slag compositions, operating conditions, system equipment, and other factors which, although elements of the process, do not constitute by themselves the present invention. Preferably the process is operated at a temperature of about 1400-1700 C., in the presence of a molten slag composed of about 0-25 percent alumina, 25-50 percent silica, 5-30 percent magnesium oxide and the balance calcium oxide, and a metallic reducing agent consisting essentially of about 50-100 percent silicon, 0-40 percent aluminum and 0-15 percent iron.

The partial pressure of the hydrogen in the condenser of the system is slightly less than the absolute pressure of the system, in that the partial pressure of magnesium in the condenser should not exceed about 10 mm. Hg. The hydrogen partial pressure is substantially less in the reactor than in the condenser, but the magnesium pressure is higher and the total pressure is about the same. The difference in magnesium partial pressure between the reactor and condenser may be small or considerable, depending upon the rate of production and the degree that magnesium diifusion controls, as well as upon thepartial pressure of the hydrogen. Other things being equal, the higher the hydrogen partial pressure, the greater the magnesium partial pressure drop.

The hydrogen in the reactor is substantially static, and the transfer of magnesium to the condenser is predominately by diffusion, as described and claimed in my application Ser. No. 26,118. In this event, the advantages of hydrogen approach the maximum, in that its high diffusivity can be given full advantage. Also, as discussed in that application, it is possible to control the level of impurities in the product magnesium by providing means to control the flow rate of hydrogen from the furnace to the condenser and to obtain a magnesium product characterized in composition by a content of metallic impurities of less than about 1500 ppm. and of silicon of between about 50-300 p.p.m. Additional means to control the flow rate of hydrogen from the furnace to the condenser are described in my co-pending application Ser. No. 143,866, filed concurrently herewith, and are shown in the figures thereof, incorporated herein by reference. Said figures also illustrate apparatus suitable for use with the present invention.

By substantially static inert gas and the passage of magnesium predominately by diffusion, I mean that the movement of the magnesium vapor is faster than the movement, if any, of the inert gas from the reaction zone to the condenser, or that the magnesium vapor passes through the inert gas, rather than vice versa. Thus, the terms are interrelated and together meet the two conditions. But these conditions are very difficult of measurement and, in part, somewhat functional. Consequently, I prefer to define the terms in a manner more precise: the molal flow rate of the magnesium vapor to the condenser must be greater than that of the inert gas, for the inert gas to be substantially static, and preferably at least twice as great; and the partial pressure of the inert gas in the condenser must be at least 0.05 atmosphere for the magnesium transfer to be predominately by diffusion, and preferably at least as high as the partial pressure of the magnesium vapor in the reaction zone. For simplicity the molal flow rate of magnesium can be considered equal to the magnesium production rate (in moles); and that of inert gas, to the recycle rate (in moles) including that amount purged.

In order that the advantages of this invention be obtained, it is necessary that the inert gas be substantially static. Of course, in any vapor system the components are never absolutely static, since the molecules or atoms are continuously moving about. By static here I mean no net movement between the reaction zone and the condensation zone. Substantially static includes as Well a net movement up to that of the magnesium vapor from the reaction zone to the condenser. If this latter rate is exceeded, the advantages of this invention are not obtained, and the magnesium vapor transfer would no longer be predominately by diifusion but by a sweeping, in which case the magnesium vapor partial pressure would be decreased. Similarly, without a substantial amount of inert gas present, i.e. a partial pressure in excess of 0.05 atmosphere, there can be no substantial diffusion but only distillation.

Also, preferably, a stream of hydrogen gas is passed through the condensation zone, in order to promote magnesium production, as described and claimed in my copending application Ser. No. 143,866, filed concurrently herewith. In this event, several advantages of hydrogen are also obtained, such as its high specific heat, if it is used for cooling the magnesium product, in which event the hydrogen is introduced at a temperature of about 100-1100" C. The hydrogen stream may be introduced near the inlet of the condenser, in the furnace or beneath the slag, or may be circulated internally, as described.

In the present invention, the use of hydrogen in the vapor space of the furnace-condenser system obtains the many advantages due to hydrogens physical and chemical properties. Hydrogen is readily available at low cost and has the highest coeflicient of diffusion of any non-ionized gas, and a relatively high specific heat. Notwithstanding that hydrogen is considered a dangerous and explosive gas, in the present system it is relatively safe, in part due to its high chemical activity and flammability, for at the temperature hydrogen exists in the present system, at least 700 C., it will readily burn if it leaks from the system. On the other hand, the oxygen of any air leaking into the furnace will react with the magnesium vapor present before any explosive mixture has a chance to form. Finally, hydrogen has the advantage of relatively high thermal stability, which renders it useful in the present system, even though high temperatures approaching 2000 C. may be encountered.

The use of hydrogen in the system of the present invention results in several unexpected advantages. First, even though hydrogen is present in the vapor space of the reactor at a partial pressure even approaching one atmosphere, the primary reaction reducing magnesium oxide to magnesium metal and the evolution of magnesium vapor from the furnace is not retarded sufficiently to block the reactor, and reasonable production rates may be achieved. Thus, the hydrogen unexpectedly does not block the reaction and evolution of magnesium vapor. Moreover, the presence of the hydrogen in the furnace system, especially as it is substantially static, results in an unexpectedly pure magnesium product. This result is due, in my view, to two reasons: first, the hydrogen requires a substantial increase in the reaction pressure, which may be achieved in the reaction systems I have described, Without a proportionate increase in the vapor pressures of the magnesium contaminants, e.g. silicon, thereby reducing their concentration in the product; and second, the hydrogen molecules in the vapor space act as a preferential barrier to mass transfer, retarding the magnesium relatively less than the contaminants. A further advantage of the use of hydrogen in the reactor vapor space is a reduction in the gas turbulence above the slag, which correspondingly reduces carry-over of contaminants to the condenser. A still further advantage of the use of hydrogen is that a stream of hydrogen may be used either to promote mass transfer, to promote the primary reaction or to promote condensation, Without encountering insuperable complications, such as fogging, which one would ordinarily expect. There are further advantages, such as economic, i.e. the clear advantages of which follow from a continuous system of operation, which also are inherent in my present invention.

The use of hydrogen gas in the present system need not be hazardous, although several precautionary measures are recommended. Preferably, unless its use for cooling is employed, the hydrogen is maintained at all times at a temperature of at least 700 C. (a reasonable maximum is 2000 C.), Which dictates the use of insulation and heat means in circulation pipes and the system as a Whole. Also, an auxiliary hydrogen system may be provided to supply hydrogen to the reactor-condenser system, should the pressure fall below one atmosphere. Alternatively, this auxiliary system could supply an inert gas such as argon or helium to the system at such times as the pressure falls below one atmosphere. As another precaution, the space around the system should be well ventilated in order to prevent the formation of pockets of hydrogen gas. Other precautionary steps are, of course, within the skill of the art.

I claim:

1. A process for the production of magnesium in a reaction-condensation system, which comprises reacting magnesium oxide with a metallic reducing agent in a reaction zone in the presence of a molten slag, evolving magnesium vapor from the reaction zone to a condensation zone and maintaining hydrogen at a temperature of about 700-2000 C. in the vapor space of said system;

wherein the molal flow rate of the magnesium vapor from the reaction zone to the condensation zone is greater than that of the hydrogen, and the partial pressure of the hydrogen in the condensation zone is at least 0.05 atmosphere; whereby the hydrogen is substantially static and the transfer of magnesium vapor from the reaction zone to the condensation zone takes place predominately by diffusion through said hydrogen; and wherein the total pressure of the system is at least about 1 atmosphere.

2. The process of claim 1, wherein the pressure of said system is about 1 to 2 atmospheres.

3. The process of claim 1, wherein the molal flow rate of the magnesium vapor is at least twice as great as that of the hydrogen,

4. The process of claim 1 including controlling the level of impurities in the product magnesium by providing means to control the flow rate of hydrogen between the reaction zone and the condensation zone.

5. The process of claim 1, wherein the reaction zone is of a temperature of about 1400-1700 C., said metallic reducing agent consists essentially of about 50-100 percent silicon, -40 percent aluminum and 0-15 percent iron, and said molten oxidic slag is composed of about 0-25 percent alumina, 25-50 percent silica, 5-30 percent magnesium oxide and the balance calcium oxide.

6. The process of claim 1 including passing a stream of hydrogen through said condensation zone.

7. The process of claim 6 including introducing said hydrogen stream at least in part in said reaction zone.

8. The process of claim 6, including introducing said hydrogen stream at least in part at one end of said condensation zone and removing said hydrogen stream at another end of said condensation zone remote therefrom.

9. The process of claim 6, including introducing said hydrogen stream at least in part through gas dispersion means located beneath said molten oxidic slag.

References Cited UNITED STATES PATENTS 3,658,509 4/1972 Avery -67 3,520,524 7/1970 Stawarz et al 75-67 UX 3,441,402 4/ 1969 Magee et a1. 75-67 X 2,362,718 11/ 1944 Pidgeon 75-67 X 2,213,170 8/1940 Peake et a1. 75-67 2,200,772 5/1940 Erdmann 75-67 3,427,152 2/1969 Eisenberg et a1 75-67 1,955,964 4/ 1934 Kemmer 203-49 X 2,095,578 10/1937 Theiler 203-49 X 1,943,601 1/1934 Hansgirg 75-67 X 3,129,094 4/ 1964 Munekata et al. 75-67 3,475,162 10/1969 Rhodes et al. 75-67 3,567,431 3/1971 Schmidt 75-67 489,363 1/ 1893 Bornholdt 203-49 1,833,717 11/1931 Laird 203-49 L. DEWAYNE RUTLEDGE, Primary Examiner M. I. ANDREWS, Assistant Examiner 

